Fluidic median selector

ABSTRACT

A fluidic median signal selector circuit including means for comparing signals, logic means, and signal-switching means, whereby the one of three input signals which is median in value is passed to the output.

United States Patent Walter M. Posingies Edina, Minn. 827,247

May 23, 1969 Dec. 7, 1971 Honeywell Inc. 7 Minneapolis, Minn.

lnventor Appl. No; Filed Patented Assignee FLUlDlC MEDIAN SELECTOR 4 Claims, 3 Drawing Figs.

US. Cl

Int. Cl Field of Search 137/815, 235/201 FlSc 1/12 137/815; 235/201 [56] References Cited UNITED STATES PATENTS 3,503,423 3/1970 Edell 137/815 Primary Examiner-William R. Cline Attorneys-Charles J. Ungemach, Ronald T. Reiling and Charles L. Rubow ABSTRACT: A fluidic median signal selector circuit including means for comparing signals, logic means, and signalswitching means, whereby the one of three input signals which is median in value is passed to the output.

PATENTEnuEc 7I97| 3,626,473

SHEET 2 [IF 2 QC L 42- -4| 90 q 62 FIG. 2

r95 us 65 8 c o s INPUT PRESSURE OUTPUT PRESSURE a l i I I I l I I E I I l l t INVENTOR. WALTER M. POSINGIES Va/J 772 ATTORNEY F LUIDIC MEDIAN SELECTOR BACKGROUND OF THE INVENTION This invention relates generally to fluid control systems, and particularly to fluidic control systems wherein control functions are performed by no-moving-part fluid devices.

Fluid amplifier devices have been used in control systems such a autopilots, engine controls, process controls, and the like. They offer the advantages of simplicity, economy. ruggedness and the ability to operate in high-temperature or high-vibration environments hostile to electronic control systems.

In such fluidic control systems, it is sometimes desirable to select one signal from a plurality of signals. For example, in a three-channel redundant autopilot control system, it may be necessary to compare the output signals of the three channels and to transmit the signal having'the median value to the system output or actuator. In such a system, the three signals are normally substantially identical, but if one of the channels should fail so that its signal goes hard over or short-circuit," then that channel becomes an extreme value and the median signal which is transmitted to'the system output is one of the remaining good signals. In this manner, erroneous signals due to system failure are prevented from reaching the output. Such triple-channel redundant autopilot systems are known in the electrical art. Applicants invention provides an all fluidic median selector circuit, so that median signal selection function can easily be performed in a fluidic control system, without the necessity of converting signals from fluidic to electrical and back to fluidic form.

Another useful application of applicants invention is in fluidic systems for the control ofjet engines. In such systems it may be necessary to select for control purposes the median signal between a signal representing a variable parameter, and high and low limits for that parameter, which are themselves variables. Applicants invention performs this function efficiently making transitions smoothly and quickly from one" signal to another when relative values change.

SUMMARY OF THE INVENTION The applicants invention provides a fluidic circuit for selecting from three fluid input signals the one having the median value, and for transmitting that median signal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of one embodiment of applicants invention;

FIG. 2 is a schematic drawing of a portion of a second embodiment of applicants invention; and

FIG. 3 is a graph of input and output signals showing the operation of applicant's invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiment of applicant's invention shown in FIG. 1

operates to select the median of fluid signals A, B and C which are applied to input means 31 through 36. The circuitry enclosed in dotted line 85 generally performs demodulation and signal comparison functions. The circuitry associated with fluid amplifiers 21 through 29 generally performs logic functions so as to generate control signals which are applied through conduits 75, 76, and 77 to the switching means which comprises fluid amplifiers 17 through 20. The switching means operates to transmit the median signal of fluid signals A, B, and C to output conduits 80 and 81, according to the control signals received. In the embodiment shown in FIG. 1, fluid signals A, B, and Care in pulse duration modulation form. A second embodiment selects the median from signals A, B, and C in unmodulated analog form. The structure and operation of these embodiments is explained in detail in the paragraphs which follow.

In FIG. 1, fluid amplifiers 11 through 20 are bistable fluid amplifiers, each comprising a power noule designated by a, first control port b, second control port c, first outlet passage d, and second outlet passage e. The bistable fluid amplifiers are configured so that the power nozzle is aligned on an axis with a splitter element which divides the two outlet passages. The two control ports are aligned on opposite sides of the power nozzle axis substantially perpendicular thereto.

The operation of a bistable fluid amplifier is as follows: Fluid from a high-pressure source applied to power nozzle a flows through the amplifier to pass out through either outlet passage e or outlet passage d. The familiar wall attachment effect causes the entire flow to attach to the wall of one of the outlet passages, according to pressure applied to the control ports. If flow is initially through outlet passage e, a pressure applied to control port 0 is effective to switch the power stream over from outlet passage 3 to outlet passage d. Likewise, a pressure applied to "control port b is effective to switch the power stream from outlet passage d to outlet passage e.

Amplifiers 21 through 29 are fluidic OR-NOR gates. Each OR-NOR gate comprises a power nozzle 0, control ports b and c, and outlet passages d and e. Outlet passage d is the OR output, and outlet passage e is the NOR output. The OR-NOR gates are configured similar to the bistable amplifiers, except that they are so designed that the power stream normally exits through outlet passage e. This can be done by geometrically offsetting the power nozzle so as to prefer flow through outlet passage e, or by means of a bias applied to an additional control port opposite control ports b and c.

The operation of the fluidic OR-NOR gate is as follows: When a high-pressure source is attached to power nozzle a, a power stream issues from power nozzle a and passes normally through outlet passage e. A pressure signal applied to control port b or c, or both, is effective to switch the power stream from outlet passage e to outlet passage :1. When the pressure signals at control ports b and c are removed, the power stream returns to outlet passage e. Thus, an output signal is seen at OR-output d whenever there is a control signal applied to control port b or c. An output signal is seen at NOR-output e whenever there is a control signal neither at b nor 0.

In' FIG. 1, a source of fluid at a pressure (not shown) is applied to power nozzles a of amplifiers 11 through 16, and 20 through 29. r

Input means 31 and 32 are provided for receiving fluid signal A, which may be the pressure difierential output of a fluid sensor or fluid amplifier, not shown. Signals A+ and A- are complementary so that when the pressure of signal A+ goes positive, the pressure of signal A- goes negative, and vice versa. Similarly, input means 33 and 34 are for receiving fluid signal B, and input means 35 and 36 are for receiving fluid signal C. In the embodiment of the applicant's invention shown in FIG. I, input signals A, B, and C are in pulse duration modulation (PDM) form.

Fluid amplifiers 11-13, fluid resistor 51-53, and fluid capacitors 54-56 together comprise demodulator means for converting fluid signals A, B, and C into demodulated analog form. Fluid signal A is applied to control ports b and c of amplifier 11 through conduits 3,7 and 38. Fluid resistor 51 and fluid capacitor 54 are connected in series to outlet passage e of amplifier 11. Amplifier 11 serves as a bufier amplifier to prevent fluid signal A at input means 31 and 32 from being loaded down by resistor 51 and capacitor 54. Positive pulses of fluid signal A+ applied to control port b of amplifier 11 cause the output of amplifier 11 toflow out passage e through resistor 51 and into capacitor 54, causing pressure to rise therein. At the end of a positive pulse of fluid signal A+, flow is switched to outlet passage d of amplifier 11 by positive pressure of complementary signal A', and capacitor 54.discharges through resistor 51 and outlet passage e. Thus, the RC network operates to integrate the pulses of fluid signal A, and the magnitude of pressure in capacitor 54 is indicative of the duration of the positive pulses of fluid signal A. In similar manner, fluid signal B is applied to control ports b and c of amplifier 12 through conduits 39 and 40, and fluid signal C is applied to control ports b and c of amplifier 13 through conduits 41 and 42. The outputs of amplifiers 12 and 13 are applied through resistances 52 and 53 and capacitances 55 and 56, respectively. in the same manner as described above, pressure signals are developed in capacitor 55 and 56 which are indicative of the duration of the positive pulses of fluid signals B and C, respectively. It will be appreciated that the pressure signals in capacitors 54, 55, and 56 represent, respectively, fluid signals A, B, and C in demodulated analog form.

Amplifiers 14, 15 and 16, together with associated conduits comprise comparison means for comparing fluid signals A, B, and C (demodulated form). Fluid signal A from capacitor 54 is applied through conduit 57 to control port b of amplifier 16 and control port of amplifier 15. Fluid signal B from capacitor 55 is applied through conduit 58 to control port b of amplifier l and control port 0 of amplifier 14. Fluid signal c from capacitor 56 is applied through conduit 59 to control port 0 of amplifier l6, and through conduit 60 to control port b of amplifier 14.

Amplifier 14 compares fluid signals B and C in the following manner: lf signal B is greater than signal C, the higher pressure in control port c will cause the power stream issuing from power nozzle a to pass through outlet passage d. If fluid signal C is greater than fluid signal B, the higher pressure in control port b causes the power stream issuing from power nozzle a to pass through outlet passage e. When flow is through outlet passage e of amplifier 14, a high-pressure signal condition exists therein and in conduit 62 to which it connects. Such a signal condition is referred to as a logical one or I." At the same time, while the output of amplifier 14 is through outlet passage e, a lowpressure condition exists in outlet passage a and in conduit 61 to which it connects. This low-pressure signal condition is referred to as logical zero or "0. Thus, when fluid signal B is greater than fluid signal C, there is a l at conduit 61 and a 0" at conduit 62, and when fluid signal C is greater than fluid signal B, there is a l at conduit 62 and a 0" at conduit 61. It will further be appreciated that since amplifier 14 is a bistable fluid amplifier, only two output conditions are possible. That is, thereis no output corresponding to B=C. lf equal signals were applied to control ports b and c of amplifier 14, the output thereof would go arbitrarily to one of its two output states, or would remain in the one it was previously in, depending upon the physical characteristics of the particular amplifier. The possibility of one or more of fluid signals A, B, and C being equal is considered in a subsequent paragraph herein.

In a manner similar to the operation of amplifier 14, amplifier produces a l at output passage d which is connected to conduit 63 when fluid signal A is greater than fluid signal B. Likewise, amplifier 15 produces a l at outlet passage e which is connected to conduit 64 when fluid signal B is greater than fluid signal A. Fluid amplifier l6 compares fluid signals A and B and produces a l at outlet passage d which is connected to conduit 65 when fluid signal C is greater than fluid signal A, and produces a l at outlet passage e which is connected to conduit 66 when fluid signal A is greater than fluid signal C.

Therefore, the net effect of amplifiers 14, 15, and 16 is to produce comparison signals on conduits 61 through 66 which are indicative of the relative values of pairs of the three fluid input signals. It will be appreciated that in place of the fluid amplifier comparison means of FIG. 1, various diaphragm of bellows devices with fluidic or mechanical pickoffs could be used to compare the pressures of the three fluid signals.

As previously described, amplifier 21 is a NOR gate, and will produce a l at its NOR output (outlet passage e) only when neither control port b nor control port c has a l present. Control port b is connected to outlet passage d of amplifier 14.through conduit 61. Therefore, a l on conduit 61 indicates that B is greater than C, while a 0" on conduit 61 indicates the converse, namely, that c is greater than b. Control port c of amplifier 21 is connected to outlet passage e of amplifier 15 through conduit 64. A l onconduit 64 indicates that B is greater than A, while a O on conduit 64 indicates that A is greater than B. A l can appear at outlet passage e of amplifier 21 only when 0s" appear at control ports b and c. In other words, a 1 can appear at the NOR output of amplifier 21 only when C is greater than B and A is greater than B. This is equivalent to saying that a 1" at the NOR output of amplifier 21 indicates that B is smaller than both A and C, or that B is low." Summarizing, a l at the NOR output (outlet passage e) of amplifier 21 signifies that B is the lowest of the three fluid signals A, B, and C, and a "0" at that outlet passage signifies that B is not the lowest of A, B, and C.

In similar manner, control ports b and c of amplifier 24 are connected to conduits 66 and 63. A 0" on conduit 66 indicates that C is greater than A, and a 0" on conduit 63 indicates that B is greater than A. Therefore, a l at the NOR output of amplifier 24 indicates that A is the lowest of signals A, B, and C. Control ports b and c of amplifier 22 are connected to conduits 63 and 62, respectively. A0 on conduit 62 indicates that B is greater than C, and a 0" on conduit 63 indicates that B is greater than A. Therefore, a l at the NOR output of amplifier 22 indicates that B is the highest of signals A, B, and C. Likewise, control ports b and c of amplifer 23 are connected to conduits 64 and 65. Since a "0 on conduit 64 indicates that A is greater than B, and a 0" on conduit 65 indicates that A is greater than C, then a l "at the NOR output of amplifier 23 indicates that A is highest of A, B, and C.

Conduit 67 applies the NOR output of amplifier 21 to control port b of amplifier 25, and conduit 68 applies the NOR output of amplifier 22 to control port c of amplifier 25. If there is 'a 0 on conduit 67 and a 0" on conduit 68, this implies that B is not the lowest and B is not the highest; therefore, B is median of A, B, and C. Therefore, a l at the NOR output of amplifier 25 indicates that B is the median signal.

Likewise, conduit 69 applies the NOR output of amplifier 23 to control port b of amplifier 26, and conduit 70 applies the NOR output of amplifier 24 to control port c of amplifier 26. If there is a 0" on both conduits 69 and 70, this implies that A is not the highest of the three signals and A is not the lowest of the three signals; therefore A is the median of the three signals. Therefore, a l at the NOR output of amplifier 26 indicates that A is the median signal. Therefore, NOR outputs of amplifiers 25 and 26 are applied to control ports c and b of amplifier 29 by conduits 7 2 and 74, respectively. A 0" on conduit 74 indicates that B is not the median signal, and a 0" on conduit 74 indicates that A is not the median signal. If neither A nor B is the median signal, then C must be the median signal. Accordingly, a 1 at the NOR output of amplifier 29 indicates that C is the median signal. Thus, the logic means of amplifiers 21 through 26 and 29 is operable to produce signals which indicate which of fluid signals A, B, or C is median.

Amplifiers 27 and 28 perform useful isolation and interlock functions, as follows. Amplifier 27 operates as an inverter. A l at control port b produces a 0" at outlet passage 2, and vice versa. The OR output of amplifier 25, which is the complement of its NOR output, is applied through conduit 71 to control port b of amplifier 27. Thus, the output at outlet passage e of amplifier 27 is equal to the output of outlet passage e of amplifier 25. Outlet passage e of amplifier 27 is connected to conduit 75. The use of amplifier 27 prevents undue loading of outlet passage e of amplifier 25, which feeds two other circuits.

The OR output of amplifier 26 is connected through conduit 73 to control port b of amplifiers 28, and the NOR output of amplifier 25 is connected through conduit 72 to control port 0 of amplifier 28. When there is a l at the NOR output of amplifier 26, indicating that A is the median signal, there is a 0" at the OR output of amplifier 26, since it is the complement, and this 0" is applied to control port b of amplifier 28. If A is median, B cannot also be median, so there is a 0" on conduit 72 which is applied to control port 0 of amplifier 28. Therefore, a l is present at the NOr output of amplifier 28, indicating that A is the median signal. lf for some reason, due to ambiguity or failure in the logic, a B is median and an A is median" signal occur simultaneously, the signal applied to control port c of amplifier 28 would prevent the A is median signal from appearing at the NOr output of amplifier 28. Therein lies the interlock function performed by amplifier 27, 28, and 29; a l signal will appear at one and only one of the NOr outputs of the three amplifiers 27, 28, and 29.

The logic means comprising amplifiers 21-29 therefore functions to produce control signals which are indicative of which of the three fluid signals A, B, and C is median in value. It will be appreciated that logic means of FIG. 1 could be modified to employ various combinations of AND and OR gates instead of NOR gates to perform the same logic function, without departing from the scope of applicants invention.

Control signals from the NOR outputs of amplifiers 27, 28 and 29 are applied through conduits 75, 76, and 77, respectively, to the power nozzles of amplifiers 18, 17, and 19, respectively. Fluid signals A, B, and C in pulse duration modulation form are applied to control ports b and c of amplifiers 17 through 19, respectively, by conduits 43 through 48, respectively. Outlet passages d of 17 through 19 are connected through conduit 78 to control port b of amplifier 20, and outlet passages e of amplifier 17 through 19 are connected through conduit 79 to control port 0 of amplifier 20. When a l signal appears on one of conduits 75 through 77, it energizes the power nozzle of the corresponding amplifier 17 through 19, so that the signals applied to the control ports of that amplifier are amplified and transmitted through the outlet passages of that amplifier, and are applied to control ports b and c of amplifier 20. The signals applied to the control ports of amplifier 20 are amplified and are passed through outlet passages d and e to output conduits 80 and 81. Thus, control signals on conduits 75, 76, and 77 are operable to cause fluid signal B, A, or C, respectively, to be transmitted through the switching means to output conduits 80 and 81.

The preceding paragraphs have described the operation of the demodulation means, comparison means, logic means, and switching means of applicant's median selector circuit. Consideration will now be given to the operation of this circuit for cases in which two or all of the input signals are equal.

It was previously mentioned that since amplifiers 14, and 16 are bistable amplifiers, their outputs will go arbitrarily to one of their two possible output states, according to the physical characteristics of the particular amplifier, when equal control signals are applied to the control ports. Since there are three comparison amplifiers, each having two possible states, eight different control signal combinations are possible at the output of the comparison means, conduits of through 66. Six of these represent reasonable statements of therelative values of fluid signals A, B, and C. The remaining two combinations represent unreasonable, impossible" statements of the relative ranks of the three signals. These two impossible conditions are, first 0'5" on conduits 61, 63, 65, land secondly, l s" on conduits 61, 63, and 65. These two conditions indicate respectively A B, B C, C A, and A B, B C, C A. Clearly, these two conditions refer to physically impossible relations between the values of fluid signals A, B, and C. However, these two conditions could occur at conduits 61, 63, and 65 due to the indeterminate operation of amplifiers 14, 15, and 16 when equal signals are applied to the control ports thereof.

These conditions present no problem to the operation of applicants invention, however, because amplifiers 27 through 29 resolve all ambiguities by insuring that one and only one of the three conduits 75 through 77 is energized. Further, if any two of the three fluid signal A, B, and C are equal or nearly identical, then clearly. one of the two'nearly identical signals will be the median of the three fluid signals, and in practical terms it matters little which of the two nearly identical signals is transmitted. Likewise, if all three fluid signals A, B, and C are equal or nearly identical, then any one of them may for practical purposes be considered the median. In such a case, the operation of amplifiers 27, 28, and 29 is such that only fluid signal B is transmitted to output conduits 80 and 81.

A second embodiment of applicant's invention is adapted to select the median of three fluid signals, the three signals being in unmodulated analog form. This embodiment consists of a modification of the embodiment shown in F IG. 1. Specifically, the second embodiment consists of the circuit shown in FIG. 1 wherein a circuitry enclosed by dotted line is replaced by the circuitry shown in FIG. 2, and wherein amplifiers 17 through 20 are proportional fluid amplifiers, rather than bistable fluid amplifiers.

In operation, fluid signals A, B, and C are received at input means 31 through 36, as in embodiment 1. However, since fluid signals A, B, and C are already in unmodulated analog form, the demodulator means of amplifiers 11, 12, and 13 of FIG. 1 are not required for the second embodiment. Therefore, fluid signals A, B, and C are applied through conduits 37 through 42 directly to the control ports of amplifiers 114, 115, and 116 which together comprise signal comparison means. Amplifiers 114, and 116 are bistable fluid amplifiers each comprising a power nozzle a, outlet passages d and e, a first pair of control ports b and c, and a second pair of control ports f and g.

Fluid signal C+ is applied to control port f of amplifier 114 through conduits 41 and 86. Fluid signal C- is applied to control port g of amplifier 114 through conduits 42 and 88. Fluid signal Blis applied to control port 0 through conduits 39 and 90, and fluid signal 8- is applied to control port a through conduits 40 and 92. Fluid signals B and C are thus applied to the control ports of amplifier 114 in opposition, so that amplifier 114 compares the relative magnitudes of the two fluid signals. A l at outlet passage d, which is connected to conduit 61 indicates that fluid signal B is greater than fluid signal C. A l at outlet passage e, which is connected to conduit 62, indicates that fluid signal C is greater than fluid signal B. Amplifier 115 compares fluid signals A and B as follows:

Fluid signal A+ is applied through conduit 37 and 94 to control port c, and fluid signal A connects through conduits 38 and 96 to control port b. Fluid signal B+ connects through conduits 39 and 91 to control port f, and fluid signal 8- connects through conduits 40 and 93 to control port g. Therefore, a l at outlet passage d, which connected to conduit 63, indicates that fluid signal A is greater than fluid signal B. Conversely, a l at outlet passage e, which connects to conduit 64, indicates that fluid signal B is greater than fluid signal A.

Likewise, amplifier 116 compares fluid signals A and C. Fluid signal A+ and is connected to control ports [and g, respectively through conduits 37, 95, and 38, 97, respectively. Fluid signal C+ and connects to control ports c and b, respectively through conduits 41, 87 and 42, 89. If fluid signal A is greater than fluid signal C, a l" will appear at outlet passage 2, which connects to conduit 66. If fluid signal C is greater than fluid signal A, a l appears at outlet passage d, which connects to conduit 65.

it will be appreciated from the foregoing that amplifiers 114, 115, and 116 perform the same comparison function performed by amplifiers 14, 15, and 16 in the first embodiment, and that the comparison signals produced thereby, at conduits 61 through 66, are likewise indicative of the relative values of pairs of the three fluid input signals. Therefore, the logic means for the second embodiment may be identical to the logic means of amplifiers 21 through 29 of the first embodiment shown in FIG. 1. 1n the second embodiment as in the first, control signals at conduits 75, 76, and 77 energize amplifiers 18, 17, and 19, respectively so as to pass the median of fluid signals A, B and C through its corresponding amplifier to amplifier 20, and on to output means 80 and 81. Since in the second embodiment amplifiers 17 through are proportional fluid amplifiers, the signal appearing at output means 80 and 81 will be proportional to fluid signal A, B, or C, whichever is median in value. By proper choice of amplifiers, or by the use of fluid resistors, the overall gain through amplifiers 17, 18, or 19 and 20 can be made equal to unity, or can be adjusted to any desired value.

FIG. 3 graphically shows an example of the operation of applicants invention. In the upper graph of FIG. 3, line '101 shows the variation in fluid signal A over a period of time. Line 102 shows the variation of fluid signal B, and line 103 shows the variation of fluid signal C, both over the same period of time. Note that lines 101, 102, and 103 show unmodulated analog signals such as might be applied to input means 31 through 36 of the second embodiment of applicants invention, or as may be found at capacitors 54, 55, and 56 of the first embodiment of applicants invention, shown in FIG. 1.

Reference numeral 104 refers to the graph of the output of applicants median selector circuit, as seen at output terminals 80 and 81. The vertical scale of a lower graph in FIG. 3 might be identical to the vertical scale in the upper graph, or it might be multiplied by a constant, according to the overall gain of the circuit. Between the time t and 1,, fluid signal A (101) is the median signal, and it therefore appears at the output. In the time interval between t, and I fluid signal B (102) is the median signal, and it appear at the output. From time 1 until time I fluid signal C (103) is median, and fluid signal A is median from time until time t For times greater than t,,, as shown in the graph, fluid signal B is median in value of fluid signals A, B, and C, and by operation of applicant's invention fluid signal B appears at the output terminals.

Other embodiments and modifications of applicants invention are possible. For example, if for a particular application of applicant's invention it were known in advance that a certain one of the three fluid signals would always be greater than one of the others, then the comparison means for comparing those two signals would not be needed, and the logic means could be simplified accordingly.

I claim as my invention:

1. A median signal selector circuit comprising in combination:

input means for receiving first, second, and third fluid signals;

signal comparison means in communication with said input means, said signal comparison means being for comparing pairs of said first, second, and third fluid signals and for producing a plurality of comparison signals indicative of the relationships between the signals of said pairs;

logic means including fluid logic elements in communication with said signal comparison means so as to receive said plurality of comparison signals, said logic means operable to produce control signals indicative of which of said first, second, and third fluid signals is median in value, according to said plurality of comparison signals; and switching means including a plurality of fluid amplifiers each having a power nozzle, said switching means connected to said input means for receiving said first, second, and third fluid signals, said power nozzles connected to said logic means for receiving said control signals, thereby to energize said fluid amplifiers so that said signal switching means is operable to transmit whichever of said first, second, and third fluid signals is median in value. 2. Apparatus according to claim 1 wherein fluid signal demodulator means are provided between said input means and said signal comparison means for converting said first,

second, and third fluid signals from pulse duration modulation form to demodulated analog form.

3. A fluid median selector circuit comprising in combinao u i r lput means for receiving first, second, and third fluid signals; 1 fluid signal comparison means in communication with said input means, said fluid signal comparison means comprising first fluid amplifier means for comparing said first and second fluid signals and for producing a first comparison signal indicative of the relationship therebetween, second fluid amplifier means for comparing said second and third fluid signals and for producing a second comparison signal indicative of the relationship therebetween, and third fluid amplifier means for comparing said first and third fluid signals and for producing a third comparison signal indicative of the relationship therebetween;

fluid logic means connected to said fluid signal comparison means for receiving said first, second, and third comparison signals and for producing in accordance therewith, fluid control signals indicative of which of said first, second, and third fluid signals is intermediate in value; and

signal-switching means including fourth, fifth, and sixth fluid amplifiers each comprising a power nozzle, control ports, and outlet passages, said control ports of said fourth, fifth, and sixth fluid amplifiers connected to said input means for receiving said first, second, and third fluid signals respectively, and said power nozzles connected to said fluid logic means, so that said fourth, fifth, or sixth fluid amplifier is energized according to said fluid control signals, whereby the median of said fist, second, and third fluid signals is transmitted to the outlet passages of the corresponding one of said fourth, fifth, and sixth fluid amplifiers.

4. The apparatus of claim 3 further including fluid signal demodulator means connected between said input means and said fluid signal comparison means, said fluid signal demodulator means including a plurality of fluid resistance capacitance networks for converting said first, second, and third fluid signals from pulse duration modulation form to demodulated analog form. 

1. A median signal selector circuit comprising in combination: input means for receiving first, second, and third fluid signals; signal comparison means in communication with said input means, said signal comparison means being for comparing pairs of said first, second, and third fluid signals and for producing a plurality of comparison signals indicative of the relationships between the signals of said pairs; logic means including fluid logic elements in communication with said signal comparison means so as to receive said plurality of comparison signals, said logic means operable to produce control signals indicative of which of said first, second, and third fluid signals is median in value, according to said plurality of comparison signals; and switching means including a plurality of fluid amplifiers each having a power nozzle, said switching means connected to said input means for receiving said first, second, and third fluid signals, said power nozzles connected to said logic means for receiving said control signals, thereby to energIze said fluid amplifiers so that said signal switching means is operable to transmit whichever of said first, second, and third fluid signals is median in value.
 2. Apparatus according to claim 1 wherein fluid signal demodulator means are provided between said input means and said signal comparison means for converting said first, second, and third fluid signals from pulse duration modulation form to demodulated analog form.
 3. A fluid median selector circuit comprising in combination: input means for receiving first, second, and third fluid signals; fluid signal comparison means in communication with said input means, said fluid signal comparison means comprising first fluid amplifier means for comparing said first and second fluid signals and for producing a first comparison signal indicative of the relationship therebetween, second fluid amplifier means for comparing said second and third fluid signals and for producing a second comparison signal indicative of the relationship therebetween, and third fluid amplifier means for comparing said first and third fluid signals and for producing a third comparison signal indicative of the relationship therebetween; fluid logic means connected to said fluid signal comparison means for receiving said first, second, and third comparison signals and for producing in accordance therewith, fluid control signals indicative of which of said first, second, and third fluid signals is intermediate in value; and signal-switching means including fourth, fifth, and sixth fluid amplifiers each comprising a power nozzle, control ports, and outlet passages, said control ports of said fourth, fifth, and sixth fluid amplifiers connected to said input means for receiving said first, second, and third fluid signals respectively, and said power nozzles connected to said fluid logic means, so that said fourth, fifth, or sixth fluid amplifier is energized according to said fluid control signals, whereby the median of said first, second, and third fluid signals is transmitted to the outlet passages of the corresponding one of said fourth, fifth, and sixth fluid amplifiers.
 4. The apparatus of claim 3 further including fluid signal demodulator means connected between said input means and said fluid signal comparison means, said fluid signal demodulator means including a plurality of fluid resistance capacitance networks for converting said first, second, and third fluid signals from pulse duration modulation form to demodulated analog form. 